Trees in the Fire-Safe Landscape

The Lafayette garden of Katherine Greenberg, showing relatively fire-resistant coast live oaks (Quercus agrifolia) planted away from the house; only vine maples (Acer circinatum) and native currants (Ribes spp.), all deciduous, are planted near the house. Photograph by RGT

On a hot, breezy, fall afternoon, a gardener leaves a weed cutter lying in the dry grass of a hillside neighborhood. He is only gone minutes when he looks back to see a wildfire, ignited from the hot engine’s manifold, racing up the hill to houses perched above. As fire engines converge on the scene, the distant sound of chainsaws can be heard. Alarmed residents across the canyon hastily cut away branches and brush in a desperate attempt to clear decades of accumulated fuel. In one garden, a panicked homeowner hacks at the spreading limbs of an old live oak. The massive branches crash to the ground. But the fire is moving too fast. The sheriff urges the owner to flee, leaving the oak limbs scattered across the grass. Days later, when the owner returns, the house is gone as are the fallen limbs, but the disfigured tree remains. He assumes the tree fed the fire and has it cut down. The oak was 176 years old.

Though that story is hypothetical, trees in the fire-safe landscape are often looked upon as the bad guys. Dramatic news videos show forests ablaze, with flames consuming massive trees in seconds. Fire safety campaigns talk about cutting, removal, and clearance. Even firefighters are trained to drop trees, cut fire lines, and remove natives. Yet, trees are a vital part of both the fire-safe landscape and the sustainable neighborhood, providing habitat for wildlife, shade for our homes and outdoor living spaces, and food for our families.

In California’s wildland-urban interface—the place where neighborhoods meet wild canyons, hillsides, and forests—the most common tree is the eucalyptus (mostly Eucalyptus globulus, E. viminalis, and E. camaldulensis). These introduced trees line roadways for miles, cover acres once home to blankets of native plants, and crowd older homes in dense hillside neighborhoods from Malibu to Marin. Developers have often favored eucalyptus for their drought tolerance and fast growth, which lends a sense of shelter to raw, newly built homes and landscapes. Critically, though, these familiar non-native trees are also highly flammable, threatening the homes they shelter. Full of volatile oils, dropping quantities of leaves, bark strips and litter that is slow to decompose, they can create a bonfire pile ready to burn. It is true that some species of eucalyptus can be highly ornamental: consider lemon-scented gum (E. citriodora) with its slender white trunks or mottlecah (E. macrocarpa) with angular silver stems. [See also Matt Ritter’s article on smaller eucalyptus in Pacific Horticulture April 2009.] Such exceptional eucalyptus may be suitable as striking individual specimens, carefully placed; groves of even these appealing species need to be dramatically thinned. Dense growing species (E. globulus et al) are best entirely removed from the landscape, replaced with native trees that are not only more appropriate for our state but also fire resistant.

In general, it is thought that broadleaf evergreens are more fire resistant than conifers. Conifers have tough thick foliage, but their high resin content ignites quickly and burns hotly. Conifers often have lots of litter, including resin-filled cones, lodged in their crowns that can catch wind-blown firebrands. Broadleaf trees have tough leathery leaves that give up their moisture slowly and will scorch before catching fire. Deciduous trees are considered even more fire safe, since their thin, moist leaves simply shrivel when exposed to extreme heat and give the flames little purchase in their canopy.

A fine blue oak (Quercus douglasii) in the wild, with a natural carpet of spring wildflowers. Photograph by Phil Van Soelen

Dependable Native Trees

Coast live oak (Quercus agrifolia) is a fine example of a fire-safe native tree. This evergreen tree is a multi-tasker in the landscape. It is drought tolerant, easy to grow, provides shade and privacy screening for your home, and offers food for dozens of native species, from wasps to woodpeckers. In nature, oaks resist fire with thick, furrowed bark and tough, leathery leaves that are slow to burn. They often have an umbrella-like canopy with draping branches and an open center of thick scaffolding limbs. Mature oaks need only be thinned of dead material within the canopy. Branches that overhang roofs or chimneys should be removed, but oaks should never be topped or coppiced.

Beneath the oaks, the most important fire safety concern is avoiding “fuel ladders,” where overgrown shrubbery is permitted to grow up into the canopy, creating a ladder that allows flames to “climb” into the crown. This is of special concern in chaparral country and in older gardens. Taller shrubs need to be thinned, spaced no closer than fifteen feet from the canopy, and cleared from underneath the trees. During a wildfire, oaks that have been cleared of fuel may scorch but will rarely burn completely; even damaged trees are quick to send out new growth from latent buds in the trunks and scaffold branches.

Ideally, oaks should be underplanted with a low carpet of native plants, such as Pacific Coast iris or Ceanothus ‘Diamond Heights’—or not disturbed at all, with a natural outer skirt of annuals, such as fiesta flower (Pholistoma auritum var. auritum), Chinese houses (Collinsia heterophylla), and miner’s lettuce (Claytonia perfoliata) in spring. An underplanting that requires summer irrigation (such as a lawn) is definitely to be avoided around native oaks, since the trees are vulnerable to sometimes lethal soil fungi that thrive with summer irrigation.

Another notably fire-safe oak is the tough blue oak (Quercus douglasii). Nurseryman and author Nevin Smith notes: “This . . . oak amazes even casual observers with the simple fact of its passage, undaunted, through the long dry season of California on sun-baked slopes and flats.” Blue oaks have an open, fountain-like habit, with branches covered in waxy, blue green leaves. They adapt to drought by maintaining a small canopy in proportion to the root mass. Blue oaks toughen their leaves with cellulose and lignin and can drop them in extreme drought situations, thus making them less likely to burn during “red flag” fire weather. The open canopy simply does not catch fire easily, as I have observed again and again during Wildland Fire School at Fort Hunter Liggett in Monterey County. There, vast patches of blue oak savannah are burned each year to train crews in wildfire fighting techniques.

Other excellent native trees for use in fire-safe landscapes are the deciduous vine maple (Acer circinatum) in the north, desert-willow (Chilopsis linearis) in the Southwest, and western redbud (Cercis occidentalis) in the interior of California, and the evergreen Pacific wax myrtle (Myrica californica) along the coast.

Many gardeners lack the space for massive oaks in their garden or may be more concerned with producing healthy, organic food for their families. Fruit trees are inherently fire safe: they actively grow in summer, have thin, moist leaves that will wilt instead of burn, and are pruned annually to remove any dead material. In wildland fires, crews are trained to use orchards as natural firebreaks, and often shelter among the rows of irrigated trees to escape runaway flames. Apples (including ornamental crabapples), pears, and even citrus can be fire safe, provided that trees are spaced openly and at least fifteen feet from any buildings. Espaliered apples or citrus can make an effective fire-safe privacy screen that provides yearround interest from flowers and fruit. Irrigate fruit trees with gray water sent from the house to mulched basins around the trees, or use rainwater collected from the roof. A rainwater collection system will also ensure that the roof and gutters are free of flammable debris.

In hot, inland areas, consider planting a specimen tree to shade the south side of your home. A handsome, deciduous shade tree, such as golden rain tree (Koelreuteria paniculata) with large clusters of yellow blooms, lantern-like seed capsules, and vibrant fall color, helps reduce energy bills in summer but will not add to your fire threat. As with any tree planting, place specimens at least fifteen feet from the home, away from overhead wires and with adequate open space between the crown and nearby trees. Other deciduous shade trees that might be considered for warmer interior regions include ginkgo (Ginkgo biloba), various ashes (Fraxinus spp.), liquidambars (Liquidambar styraciflua), and Chinese pistache (Pistacia chinensis).

Forested Settings

In more densely forested communities, such as the Oakland hills, Santa Barbara, or Mill Valley, consider creating an open, healthy, and welcoming space within the suburban forest. This does not mean clear-cutting. Rather, consider the growth habit of each tree to maximize its natural beauty. Around the home, most canopy trees should be limbed up or thinned to at least fifteen feet from the ground. Prune away any branches that overhang roofs and chimneys. Farther away from any buildings, clean away dead branches and tangled growth to create an open, dappled shade that encourages new saplings to grow. Space trees evenly, removing those that are thin, sickly, or crowded, but avoid clearcutting the areas around the home. Pick up woody litter but allow fallen leaves to decompose in place, nourishing the trees in a thick, natural mulch. On bare soil under established trees, add a thick layer of chopped and composted leaves to retain soil moisture and improve the soil.

Where privacy is a concern, remember the “rule of thumb:” hold your thumb at arm’s length and place it in front of the scene you wish to block, perhaps your neighbor’s kitchen window. Plant a well-chosen tree in that spot, rather than a perimeter row of trees that could act as a “fuse” connecting your home to any approaching flames. You will save both work and money, while keeping your views private and your home safe from fires.

Though some may see trees as a threat, when carefully chosen and placed, trees really are assets in the landscape and, when kept clean and healthy, are fire safe. Native and fruit trees are the best choices for the garden; they enhance our living environment by sheltering wildlife, providing shade and fruit, and adding to the beauty of our homes. When we balance our needs with the reality of wildfire, we can provide safe neighborhoods for our families and defensible spaces for the brave firefighters who defend our homes.